Until now, North Texas has been one of the least likely places in the country to have an earthquake.

But after the Dallas area suffered a series of more than 120 quakes since 2008, the U.S. Geological Survey is re-evaluating the metroplex’s “seismic hazard” — or the risk of experiencing earthquakes.

This year, for the first time, the USGS will include quakes believed to have been caused by human activity in its National Seismic Hazard Map, which engineers use to write and revise building codes, and which insurers use to set rates.

The map predicts where future earthquakes will occur, how often they will occur and how strongly they will shake the ground.

The USGS probably will dial up the earthquake risk for the Dallas-Fort Worth area, though the increase may be small, said Mark Petersen, chief of the agency’s National Seismic Hazard Project in Golden, Colo.

Scientists have linked two local earthquake clusters with wastewater injection wells, where companies dispose of fluids used in the hydraulic fracturing process. One cluster began in 2008 near Dallas/Fort Worth International Airport. The second occurred near Cleburne in 2009.

Researchers are investigating two more recent clusters: one in Azle and another in Irving that continued Saturday with a 2.2-magnitude quake that was reported about 8 p.m.

Petersen said the updated version of the map would be published before the end of the year.

The increase in tremors, which is also happening in Oklahoma, Colorado and other parts of the country, is changing the way seismologists think about earthquakes.

Between 2010 and 2013, people living in the central and eastern U.S. felt five times as many quakes per year, on average, as they did between 1970 and 2000. Scientists have linked many of these earthquakes to human activities, including parts of the oil and gas production process.

While larger quakes are unlikely in North Texas, scientists can’t rule out the possibility. In 2011, a 5.7-magnitude temblor struck near injection wells in Oklahoma, causing widespread damage.

New tools needed

USGS scientists have had to develop new tools to study man-made, or induced, earthquakes.

The current seismic hazard map, which was released in 2014, bases a locality’s earthquake risk on how many quakes the area has sustained over hundreds of years. Based on those findings, scientists forecast a region’s chance of having an earthquake over the next 50 years.

But induced earthquakes strike and vanish over much shorter time scales.

“It’s very difficult to assess where induced seismicity might occur,” Petersen said. “They’re based on economic and policy decisions which are difficult to forecast, especially over a 50-year time period.”

So, starting this year, the USGS will publish the map annually instead of every six years. “It may even be more frequent than that,” said Petersen.

A massive effort is underway at the USGS to understand how man-made quakes behave differently from natural ones.

“There really aren’t any distinguishing features on the seismogram that tell us where an earthquake came from,” said Bill Ellsworth, a USGS seismologist in Menlo Park, Calif., who studies induced quakes. “They’re releasing the stored energy in the earth whether they’re natural or induced.”

Unusual pattern

Susan Hough, a USGS researcher in Pasadena, Calif., turned to reports from the public to help identify differences.

A few years ago, she noticed an unusual pattern as she analyzed a handful of earthquake sequences that had been linked with wastewater injection wells.

She saw that the intensities of the earthquakes were significantly smaller than expected for natural quakes of a similar magnitude. The USGS measures intensities based on citizens’ reports collected in the USGS “Did You Feel It?” database.

Last year, she published a paper examining 11 of the largest quakes that studies have linked with human activity. The report included three in Texas — one in East Texas and two in South Texas.

She found that induced earthquakes create less shaking than natural quakes 6 miles from the epicenter and beyond. Closer to the epicenter, induced quakes cause about the same amount of shaking as natural quakes, and sometimes more.

“These induced earthquakes are just systematically different,” she said.

She believes the differences stem from the fact that induced quakes tend to be shallower than natural quakes and release less energy as the fault slips.

There are two possible reasons they may release less energy, Hough speculates. First, less stress has had time to build up in the fault.

“A natural fault will sit there and build up and build up and build up,” said Hough. “And when it finally breaks, there will be a certain stress level that it’s releasing. But if you nudge that fault to go sooner, it hasn’t built to the breaking point already.”

Second, fluid from an injection well may help push the fault apart so that it slips more smoothly and with less friction.

The Azle and Irving quakes, which researchers are still studying and have not linked with human activities, follow the same pattern as other induced quakes, Hough said.

‘Not a smoking gun’

Not all scientists agree with Hough’s interpretation.

Cliff Frohlich, a seismologist at the University of Texas at Austin who studies the North Texas earthquake clusters, said the differences Hough spotted may have little to do with the earthquakes’ cause.

Shallower earthquakes are not as widely felt as deeper quakes because rock in the upper layers of the earth’s crust is softer and more varied. Seismic waves scatter and weaken as they pass through it.

Induced quakes tend to be shallower because fluids injected into the ground reach depths of only 1 to 3 miles. Most natural earthquakes occur at depths between 3 and 10 miles, though occasionally some are shallower.

The USGS is factoring some of Hough’s findings into a new ground-shaking model that Petersen and his colleagues will incorporate into the new map. The model takes depth into account for the first time.

Scientists also are comparing the frequency with which man-made and natural earthquakes occur. Earthquakes can strike once, as a short series of events, or as a drawn-out sequence, like the one in Irving, which started in April and has generated almost 40 episodes of ground shaking.

“The way we describe earthquakes is a bit like an epidemic,” said Ellsworth. “One earthquake has the potential to trigger others.”

There is some indication that human-induced swarms may contain more quakes than natural ones. Ellsworth is studying this phenomenon, and his findings may be incorporated into the new hazard map as well.

“It’s not a huge difference,” he said. “It’s not a smoking gun as a way of discriminating one type of earthquake from another.”

Building impact

Once the new hazard map is out, public safety officials and engineers will begin grappling with how it might affect building codes around the country.

A key problem is that it can take years to update and adopt new building standards.

“By the time we’ve made a map for a building code and that building code gets passed into law, the seismology may be significantly different than it was when we made the map,” Petersen said. “It’s difficult to know how to deal with that.”

Jim Sealy, a Dallas architect, chairs the board of the National Building Seismic Safety Council for the National Institute of Building Sciences. The council uses funding from the Federal Emergency Management Agency to develop provisions to be included in the International Building Code, which most states, cities and counties follow, Sealy said.

The recent quakes in Texas have not fazed him, and he believes the current codes are adequate. “If we start seeing signficant damage to buildings, that will be the time to investigate changing the way we do things in this area,” he said.

However, any change would affect only new construction — and possibly buildings such as hospitals that must stay open during emergencies. Changes to most existing buildings would be cost-prohibitive, he said.

“The earthquakes have been minor,” Sealy said. “I felt one here in my office, and it felt like someone came in behind me and moved my chair. There’s been a lot of hype about this.”